Medicament delivery devices as e.g. injectors for injecting medicament into the tissue of patients have become widely used, and in particular since these injectors have facilitated for larger groups of patients to self-administer their drugs, due to the functionalities of the injectors.
Examples of functionalities of the injectors are auto-penetration and/or auto-injection. A common design for auto-injection that could also be used for the preceding penetration is to have a loaded compression spiral spring acting on a plunger rod, which in turn is acting on a stopper arranged in a medicament cartridge or syringe. When the spring is released, a high force will be acting on the plunger rod and thus the stopper and the body of the syringe. Often the spring is at very high tension, particularly at the beginning of its stroke, why forces applied may be considerable. It is not uncommon that the stopper has adhered to the inner wall of the cartridge or syringe, for example due to the material properties of the stopper and long time storage. In order to handle this, the spring has to be dimensioned such that it has the necessary force to release the stopper.
This in turn, a well as the sudden impact of the plunger rod on the stopper, increases the risk of breaking the cartridge or syringe that often is made of glass. This situation has to be avoided, because if it occurs, the patient may not be able to receive any medication, which might be fatal.
One solution to this problem is disclosed in WO 00/24441 describing an auto-injector arranged with penetration/injection dampening means. According to one embodiment the damper comprises a stator part attached to the injector head (plunger rod) and a toothed rotor wheel engaging a toothed rail. During movement of the injector head the rotor wheel is also moved, and a retarding force is obtained between the rotor wheel and the rail.
According to another embodiment, the damper is a viscous damper with a piston arranged in a cylinder in the injector head. During movement oil in the cylinder passes flow restrictions in the piston, thereby creating a dampening action.
However, both embodiments disclosed in WO 00/24441 are not very feasible. The rotational damper is rather bulky and comprises many components. The viscous damper requires seals so that the oil cannot escape into the injector, which could be a problem when storing the injector for longer periods. Also, many types of oils are not compatible with many types of polymers, thereby risking degeneration of the components of the injector.
Document GB 2 414 404 discloses an injector arranged with a dampening means comprising a compartment in a second part of a plunger rod comprising highly viscous fluid and a first part of the plunger rod releasably attached to the second part. When the plunger rod has moved a certain distance during penetration and injection, the first part is released and then acts on the fluid in the compartment, whereby the fluid is forced through a vent bore, dampening the movement of the first part relative the second part. The aim for this is to delay the release of the syringe to allow remaining content of the syringe to be discharged before syringe is released. The delay is used to compensate for any stacking of tolerances in the injector.
Document WO 031097133 intends to solve the same problem as GB 2 414 404, but instead of a viscous fluid, a compartment in the front part of a plunger rod contains air, which air is pressed through a small aperture by a second part of the plunger rod acting on the compartment.
None of the documents GB 2 414 404 or WO 031097133 really disclose a dampening function but rather a delay function ensuring that an injection is completed before the needle is automatically withdrawn from the injection site.